1. Field of the Invention
The present invention relates to a synchronization establishment circuit and synchronization establishment method for generating internal signals that synchronize with external signals provided at a predetermined cycle. The present invention can be applied to the synchronization establishment between a base station and terminals in a radio network, such as a beacon network.
2. Description of the Related Art
A beacon network is known as a type of radio network. As FIG. 9 of the accompanying drawings shows, the beacon network 900 is comprised of one base station 910 and many terminals 920, 920 . . . . The base station 910 and each terminal 920 are connected wirelessly.
In the beacon network 900, synchronization must always be established between the base station 910 and each terminal 920. Therefore in the beacon network 900, data having a predetermined format, called a beacon, is regularly sent from the base station 910 to each terminal 920. Each terminal 920 can know the timing when the base station 910 sends a beacon (i.e., data transmission timing), and the relationship between the data transmission timing and the data transmission/reception enable period by analyzing the data in the received beacon. Based on this information, each terminal 920 transmits/receives data to/from the base station 910.
FIG. 10 and FIG. 11 of the accompanying drawings depict the operation of the base station 910. FIG. 10 is a timing chart, and FIG. 11 is a flow chart.
As shown in FIG. 10 and FIG. 11, the base station 910 sends one beacon at each beacon interval T_Beacon. After a beacon is sent (see step S1101 in FIG. 11), the base station 910 enters transmission/reception enable status (see step S1102 in FIG. 11). When the period T_ON elapses from the transmission of the beacon (see step S1103 in FIG. 11), the transmission/reception enable status ends, and the base station 910 stops (see step S1104 in FIG. 11). Then the period T_Beacon elapses from the transmission of the beacon (see step S1105 in FIG. 11), and the next beacon is sent from the base station 910 (see step S1101 in FIG. 11).
Such information as the beacon interval T_Beacon and transmission/reception enable period T_ON is included (carried) in a beacon. Each terminal 920 must analyze the information included in the beacon, and perform internal control to communicate with the base station 910 within the transmission/reception enable period. In order to stably perform network communication, control of the operation of each terminal 920 must be synchronized with the operation of the base station 910 at high precision using the beacon interval T_Beacon.
Synchronizing the base station 910 and the terminal 920 at high precision makes the cost of the beacon network 900 enormously expensive. To implement synchronization at high precision, the clock precision of the base station 910 and the terminal 920 must be matched strictly. Thus, a clock generation circuit with extremely high precision must be installed for each of the base station 910 and the terminal 920. This shortcoming becomes more conspicuous as the beacon interval T_Beacon becomes longer. This is because as the beacon interval T_Beacon becomes longer, the influence of the difference of the clock precision between the base station 910 and the terminal 920 becomes significant. For example, when the room temperature in a factory or the temperature of a refrigerator is managed by a beacon network 900, the beacon interval T_Beacon is set to be very long, since management in a short cycle is not only necessary but also increases the power consumption of the terminal. However, if the beacon interval T_Beacon is very long, the synchronization shift between the base station 910 and the terminal 920 becomes very large.
Use of the same clock for both the base station 910 and the terminal 920 is disclosed in Japanese Patent Kokai (Laid-Open Application) No. 2003-60652, for example. In this document, the PLL circuit in the terminal generates clocks for synchronization using the cycle of the beacon received from the base station. A clock for synchronization in the terminal is generated based on the clock of generating the beacon in the base station, so that the base station and the terminal can be synchronized at high precision.
However, Japanese Patent Kokai No. 2003-60652 must use a PLL circuit. Thus, the control circuit scale is large and cost is high.